Matt Jones

**M.Ed., George Washington University**

Dept. chair at a high school

Matt is currently the department chair at a high school in San Francisco. In his spare time, Matt enjoys spending time outdoors with his wife and two kids.

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An object is in **free fall** when gravity is the only force to move it through space. In reality, **free fall** is affected by variables such as wind iance, but when physicists discuss **free fall**, they generally assume that it is taking place in a vacuum. The acceleration of an object in **free fall** is 9.8 m/s^2.

So what about a free fall, you know what a free fall is, here's a ball I let it go and then it accelerated towards the ground right? While essentially free fall is an object moving only under the force of gravity, now in reality that tennis ball there was two forces was gravity pushing it down and it was also friction or wind resistance pushing up but when we calculate free fall we're only going to use gravity we're going to pretend that we're dropping that tennis ball in a vacuum or accelerating in space where there's no resistance okay? So let's review remember gravity, the force of gravity it is a force is 9.8 meters per seconds squared it is the change in velocity over time right? And that's what happening as that ball moves down it is increasing in velocity okay? So we're going to need to use gravity to solve these equations. Typically, in a Physics class you'll be asked to solve two types of equations when calculating free fall.

The first one is the velocity of an object at a certain point in time after it's dropped from rest. The second is the distance that's it's falling after it's been dropped, so we'll calculate those two.

The first formula you need to know to calculate the velocity of an object after it's fallen is it's going to be force of gravity times time okay? Let's look at a example of a question you might get okay? Free falling object dropped from rest so we're not accelerating it down, we're just dropping it from rest, what is the instantaneous velocity, the velocity exactly at 3.0 seconds okay? After it's been dropped okay? Well we're going to use this formula and we're going to say gravity is 9.8 meters per second squared times 3 seconds okay? And my seconds, if I have a second in the numerator and a second squared in the denominator I can simplify that and say that is going to be removed and I'm going to just make this meters per second and when I calculate that out and I'm going to get my result is 29.4 meters per second remember to always carry those units and simplify so that you know that you have the proper units okay? And this case what they're asking from instantaneous velocity and that 29.4 meters per second is your instantaneous velocity okay?

Lets look at the second problem okay? Well, I want to know how far did that object fall in those 3 seconds? So I need to use another formula for that which is one half the force of gravity okay, times time squared, cause remember we're talking about a change in velocity so that change is going to keep increasing every second. So if I plug this equation into this question, we've got at that 3 seconds okay, we got one half times the force of gravity 9.8 meters per second squared times 3 seconds squared. If I say 3 seconds squared, what is that equal? That equals 3 squared which is 9 and second squared, so that equals 9 seconds squared okay? So I'll put 9 seconds squared in there okay? And again units can trip you up and units can get confusing but I've got second squared in the denominator and second squared in the numerator so I can cancel that and cancel that and I have one half times 9.8 times 9 seconds and that equals 44.1 meters and that's the distance that the object has fallen in that 3 seconds and that 3 seconds that is the velocity and those are two types of equations you need know how to solve in calculating free fall.